In general, a control of the air-fuel ratio in a gas engine is set corresponding to a fuel gas having a fixed composition, but actually, a composition of a supplied fuel gas is not fixed.
In a conventionally disclosed configuration, an A/F valve and a solenoid valve are arranged in a fuel path, and a control is performed such that the average opening degree of the solenoid valve is settled to 50%, to perform stoichiometric combustion in a gas engine fueled by a gas having a variable calorific value (for example, see Patent Literature 1 (PTL 1)).